High specific activity labeled ligands for neurotransmitter receptors can be used to characterize in vivo the status of the receptor. With an ideal ligand one can study regional distribution, saturability and pharmacological specificity of the binding and characterize the various functional states of the receptor. To study DA receptors in vivo, we have selected spiroperidol. This drug is an almost ideal probe for binding studies in vivo because it is poorly metabolized in brain and binds tightly to the DA receptors. The data obtained in animal models could be extended to measure DA receptor function in man injecting an appropriately labelled spiroperidol that could be used advantageously for ECT scanning.